I have managed to connect to the server from my Soundbridge (and from an iTunes on Mac computer), but when I browse (on Soundbridge) or click on the shared library (on Mac’s iTunes), there is no song whatsoever.

I have also managed to open the web configuration page (I keep the default port 3689).

Please help me!

Can somebody lay out a step-by-step guide on how to clean-install Firefly on Dapper?

What OS are you running firefly on?
It seems that firefly doesn’t load its plugins, hence doesnt understand any files.
Check your config file, especially the plugin_dir, and see if firefly’s user has the right permissions.